1. Field of the Invention
The present invention relates to the production of methacrylic acid and methacrolein by means of a gas phase oxidation of isobutane with a molybdenum heteropoly acid catalyst.
2. Discussion of the Background
Methacrylic acid can be made by gas phase oxidation of various starting materials, such as isobutylene, methacrolein, isobutyric acid, and isobutane. The oxidation reactions with molecular oxygen are managed selectively on solid-body catalysts, retaining the molecular carbon arrangement. These solid catalysts, almost without exception, contain molybdenum in an oxidic bond as an essential constituent, along with other elements that influence the activity and selectivity of the catalysts. Particularly important are catalysts that are made or built up on the basis of heteropoly acids of molybdenum with phosphorus as the central atom.
According to DE-OS 27 22 375, one may, for example, use H.sub.5 PMo.sub.10 V.sub.2 O.sub.40 -heteropoly-acid-containing catalysts, among other things, as well as copper-containing catalysts, both for the oxidation of methacrolein into methacrylic acid, and for the oxydehydration of isobutyric acid or its esters into methacrylic acid or its esters.
According to EP-B 0 005 769, catalysts having the formula Mo.sub.a V.sub.b P.sub.c X.sub.d O.sub.e, which have a heteropoly acid structure, are used for the oxidation of isobutylene and/or tertbutanol into methacrolein and methacrylic acid. A process for the oxidation of methacrolein into methacrylic acid with catalysts of the same kind is described in DE-OS 30 10 434.
The heteropoly acid having the formula H.sub.8 PMo.sub.10 VO.sub.39 and its anhydride PMo.sub.10 VO.sub.35, according to EP-B 0 194 541, are suitable as more temperature-stable heteropoly acid catalysts in oxidation reactions, such as oxidation of the olefins propylene and isobutylene into the corresponding unsaturated aldehydes acrolein and methacrolein, and also their further oxidation into acrylic acid and methacrylic acid, and in oxydehydration reactions of isobutyric acid or its esters into methacrylic acid or its esters.
According to EP-B 0 113 084, copper derivatives of this heteropoly acid anhydride, for example, Cu.sub.0.2 PMo.sub.10 VO.sub.35.2 and also copper derivatives of other heteropoly acids, such as H.sub.5 PMo.sub.10 V.sub.2 O.sub.40 -heteropoly acid, are very selective catalysts in the oxydehydration of isobutyric acid into methacrylic acid.
EP-A 284 947 mentions the manufacture of molybdenum heteropoly acid oxidation catalysts of the kind described above. Here, one uses water-soluble, practically non-volatile organic compounds, especially polymers, which are then calcined at 150.degree.-400.degree. C. in the presence of oxygen. Catalysts made in this fashion stand out by their improved long-term behavior in the course of oxydehydration of isobutyric acid into methacrylic acid.
Furthermore, EP-A 0 376 177 proposes a process for making methacrylic acid by oxidation of methacrolein on oxidic catalysts having the formula MO.sub.12 P.sub.a V.sub.b Cs.sub.c As.sub.d Cu.sub.e X.sub.f Y.sub.g O.sub.x. During the shaping of the catalyst, one can add known carbon-containing compounds as slip additives. In addition, one can add forming expedience and reinforcing agents, such as inorganic microfibers (for example, glass or asbestos). The catalysts are calcined at temperatures of 180.degree. to 480.degree. C., possibly in an air atmosphere.
Catalysts for selective oxidations generally have a small inside surface; i.e., the catalyst grain is provided with relatively few pores, or they are made with porous carriers with wide pores in the interior of the carrier material in which the catalytic material is then embedded. Such carriers are very permeable for a diffusion stream for the transport of matter and energy to the catalytically active material. Combinations of substances containing molybdenum, vanadium, phosphorus, and oxygen with carriers having a porosity of 10 to 80% and an interior surface of less than 1 m.sup.2 /g are described as catalysts for the oxydehydration of isobutyric acid into methacrylic acid in DE-OS 31 45 091.
Finally, DE-OS 41 13 423 describes oxidation catalysts with molybdenum, phosphorus, and vanadium as essential elements in an oxidic form and on a heteropoly acid base which contain channels formed by burning out organic fibers. They are used for the oxidation of propylene or isobutylene into acrolein or methacrolein, for their further oxidation into the corresponding (meth)acrylic acid, and for the oxydehydration of isobutyric acid or its lower esters into methacrylic acid or its esters.
The oxidation of isobutane into methacrylic acid and methacrolein on oxidic solid catalysts that contain molybdenum, antimony and phosphorus is known from EP-A 0 010 902, according to which one gets, with approximately 10% isobutane conversion, methacrylic acid with approximately 50% selectivity and methacrolein with about 20% selectivity. According to JP 0 320 237, isobutane is converted into methacrolein with 9.5% conversion in 51.4% selectivity on an oxide contact with phosphorus, vanadium, molybdenum, antimony and copper.
Solid catalysts made of heteropoly acids of molybdenum and their salt derivatives are also suitable for selective isobutane oxidation. According to EP-A 0 418 657, these are phosphomolybdic heteropoly acid salts that forcibly contain at least one element of rubidium, cesium and thallium. Optionally, these heteropoly acid salts may forcibly contain vanadium, arsenic, copper or another metal.
EP-A 0 425 666 describes a process for making methacrylic acid and methacrolein by oxidation of isobutane, performed on catalysts made from molybdenum heteropoly acids with phosphorus and/or arsenic as the central atom. The catalysts forcibly contain at least one constituent from the group of an alkali metal, an alkaline earth metal and thallium, plus vanadium and/or copper, and optionally, additional metallic elements.
Catalysts for isobutane oxidation made from heteropoly acids of molybdenum with phosphorus and/or arsenic as a central atom and without vanadium as a molybdenum-replacing coordinating periphery atom are described in Japanese patent application JP 02 42 033, according to which copper is also contained therein obligatorily, plus JP 02 42 034 and JP 62 132 832, as well as JP 63 145 249, with the last two applications mentioned covering variations of the process.
The attainable yield of end product, the conversion of a starting material and/or the attainable space-time yield are important quality characteristics of a catalyst, in conjunction with a certain conversion. However, the selectivity (i.e., the quotient of yield and conversion) of a catalyst indicates what part of the starting material was actually converted into the desired end product. As a result, the selectivity is considered the most important quality characteristic of the catalyst. Prior approaches to the oxidation of isobutane into methacrylic acid and methacrolein left considerable room for improvement in selectivity, and therefore, were deficient with regard to the selectivity of the catalyst and process.